Conventionally, as disclosed in JP-U-2-132515, for example, an ultrasonic sensor is provided in a vehicle door. The ultrasonic sensor detects the distance from the vehicle door to an obstacle for protecting the vehicle door from the obstacle when the vehicle door is opened. The ultrasonic sensor provided in the vehicle door emits an ultrasonic pulse perpendicularly to an outer surface of the vehicle door and accepts the ultrasonic pulse reflected from an obstacle. Thus, the ultrasonic sensor detects the distance from the vehicle door to the obstacle. In general, one ultrasonic sensor is capable of detecting an obstacle in a limited area, which is insufficient for including the entire vehicle door. Therefore, in JP-U-2-132515, one vehicle door is provided with multiple ultrasonic sensors for detecting an obstacle throughout a large area of the vehicle door. However, cost of the entire apparatus may significantly increase due to provision of a large number of sensors.